There is currently one rotation per year for Standard. It is the first friday of October. At that time, the new large fall set becomes legal, the oldest block rotates out, and the previous year's core set rotates out. Since the new core set of each year comes out and becomes legal during the summer, there is a period of a few months where two core sets are legal at the same time.

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I'm a lvl 1 judge, but that doesn't make my answers more valid than those of people who aren't judges. What matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.